1. Field of the Invention
The present invention relates to a timing pulse generator for generating timing pulses synchronized with a horizontal synchronizing signal which is contained in a TV video signal or another video signal for CRT display. Such timing generator may be used in a superimposing apparatus which superimposes display signals of letters, characters or the like on a video signal and displays them together with a video image on a cathode ray tube. One example of such superimposition is to display the receiving channel number or sound level on a TV program display. Another example is a teletext in which selected one among a plurality of information transmitted from a broadcasting station is superimposed in a TV display transmitted under control of a receiver.
2. Description of the Prior Art
A prior art timing pulse generator comprises a generator of clock pulses synchronized with a horizontal synchronized signal which is separated from the video signal, the clock pulses having a frequency of 300 to 400 times of the horizontal synchronizing signal, and a counter counting the clock pulses to define the superimposing position on one scanning line. The position corresponding to the predetermined numbers of the clock signal is the superimposing position. The counter is reset by the horizontal synchronizing signal to recount the clock pulses to determine the superimposing position on the next scanning line. The generator further comprises a timing generator which counts the clock pulses and produces a timing signal such as a readout timing signal to be applied to a memory which stores display elements of the letters or the characters to be displayed on a CRT in superimposition of a video image. The timing generator is also reset by the horizontal synchronizing signal to produce a next timing signal.
As mentioned above, the counter and the timing generator are reset by the horizontal synchronizing signal as a gate pulse in the prior art. If the horizontal synchronizing signal includes a spike noise or does not have a sufficient waveform, resetting of the clock counter and the timing generator becomes imperfect, and the counter miscounts the superimposing position and the timing generator misreads the memory, resulted in a disturbance of superimposed letters or characters.